Honest John's top 20 questions

Can you be fined for flashing oncoming cars to warn them of a speed trap? I
seem to recall you mentioning that the police have to prove the drivers they
flashed were actually breaking the speed limit (or about to break it).

DB, via email

Correct. If they issue a fixed penalty and the driver accepts it, then that's
effectively pleading guilty whether or not an offence has been committed. If
it went to a proper court, case law (Glendinning) has held it not to be an
offence unless the police can prove that the drivers flashed were actually
speeding.